August 18, 2000
Genetic Program Builds the Pipeline that Nourishes Tumor Growth
Detailed analysis of the activity levels of thousands of genes from
blood vessel cells isolated from normal and cancerous tissue has led to
the identification of a genetic program that constructs the pipeline
that supplies blood and nutrients to colorectal tumors.
Although the research was done primarily in cells derived from
patients with colorectal cancer, the researchers believe that this
genetic script may be a common one that runs when tumors need to supply
themselves with the factors needed to sustain growth. The findings were
reported in the August 18, 2000, issue of the journal Science by
a research team that included Howard Hughes Medical Institute
investigator Bert Vogelstein, Kenneth Kinzler and Brad St. Croix, who
are at The Johns Hopkins University Oncology Center. The Hopkins team
collaborated on the research with scientists from Duke University
Medical Center.
“We were delighted in the sense that many of these 46 genes can now become reasonable targets both for research and perhaps for new diagnostics and therapeutics.”
Bert Vogelstein
"For close to twenty years we have been studying primary tumors from
patients," said Vogelstein. "What’s new is that we are now
studying the non-neoplastic, or non-cancerous, components—in this
case the endothelial cells on which tumors depend for their blood
supply and nutrients. The idea that these cells are critical to tumor
growth was first suggested by Judah Folkman at Harvard," said
Vogelstein.
The researchers started their studies by purifying endothelial cells
from normal and cancerous tissue of patients. The researchers next
analyzed the amount of messenger RNAs (mRNAs) produced by genes
expressed in endothelial cells. The mRNA level of each gene gives the
researchers a good estimate of the activity of each gene.
The scientists used SAGE (serial analysis of gene expression), a
technique they invented to determine the activity level of genes. In
SAGE, the enzyme, reverse transcriptase, is used to produce
complementary DNA from the mRNA derived from cells under study. The DNA
is then snipped at a defined position, creating a unique identifier
"tag" that corresponds to a single gene. The researchers can then
analyze the number of unique tags present in their sample and deduce
how much mRNA exists for each gene.
The researchers generated about 100,000 tags from endothelial cells
in normal and cancerous tissue. These tags corresponded to more than
32,500 unique mRNA transcripts. Many endothelial cell genes showed
unique patterns of activity in one or the other type of tissue. The
researchers were intrigued by a group of 46 genes, which they called
tumor endothelial markers (TEMs), because these genes were elevated
tenfold or more in tumor endothelium.
"We were really surprised at the number of differences we found,"
said Vogelstein. "And, we were delighted in the sense that many of
these 46 genes can now become reasonable targets both for research and
perhaps for new diagnostics and therapeutics."
The researchers found that at least seven of the 46 genes appear to
be involved in forming or extending the scaffolding that cells use in
creating new blood vessels. Thus, these genes are likely to be
necessary for the growth of new blood vessels produced by tumors, said
Vogelstein.
To validate that these genes were, indeed, activated in other types
of cancer, the researchers measured the activity of nine of the genes
in two other patients with different forms of cancer—finding,
again, that these genes were expressed in high levels in tumor cells,
but not in normal cells.
The activity levels of a select group of TEMs in cancers of the
bone, liver, lungs, pancreas and brain were significantly higher than
the activity levels of those genes in normal endothelial cells. "This a
very promising finding because it means that if these genes become
therapeutic targets, for example, by creating antibodies against their
proteins, those antibodies would not only be potentially useful for
colon cancers, but probably for most cancers," said Vogelstein.
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